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McGuffin and Gottesman: Found 87% concordance rate between MZ twins and 72% between
DZ twins for aggressive and anti-social behaviour, supporting genes to explain aggression because
there is a higher concordance rate between MZ twins who share 100% of their genes. However
the concordance rate is not 100% so the environment must have a role. Also twins are likely to
have the same upbringing and are likely to be exposed to the same kind of things such as
aggression; perhaps this could explain the high concordance rate rather than genetics, it is very
difficult to isolate genes and the environment they interact with one another. Thirdly is all
anti-social behaviour aggressive? Playing loud music is considered anti-social but it is not
aggressive...difficult defining aggressive behaviour.
Mason and Frick: meta-analysis of 12 studies and 3795 twins. 50% variance between
non-anti-social and anti-social behaviour was due to genetic factors. More of a genetic influence
was found for the more violent behaviour. Suggesting a strong genetic link between genes and
violence, however in a meta-analysis the researcher does not know the individual studies and can
never be sure how reliable and valid each study individual study was, meaning the final
meta-analysis may be flawed as it may have used flawed data.
Coccoro et al: Studied aggressive behaviour in 182 MZ twins and 118 DZ twins they found that
genes accounted for more than 40% of the individual differences in aggression. Show support for
a genetic link however they also found the environment accounted for 50% of individual
differences in physical aggression and 70% for verbal aggression. This suggests that the
environment plays a more important role than genetics particularly for verbal aggression. Language
is picked up quicker and more readily that physical aggression. This may be due to the SLT or mirror
neurons where they are copying their parents.
Hutchings and Mednick: studied 14,000 adoptions and found a significant positive correlation
between the violent criminal convictions of a son and their biological father. Large sample
increasing the reliability, and suggesting a genetic link. However it may not be due to just genes
sons may be aggressive as a result of feeling abandoned by their fathers and having resentment
towards their biological parents. Biological parents who put their children up for adoption are
more likely to be aggressive than average member of the public.
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Gene for aggression
There is no evidence to suggest a gene for aggression but MAOA reduces the level of serotonin in
the blood and is linked with aggression and depression.
Brunner: Researched male members of a Dutch family with very high incidences of violent crime
including rape, murder etc. He found that they had a marker on their X chromosome which was
linked to MAOA. Urine samples revealed high level of noradrenaline, dopamine and